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Object Scopes

Object scope is a configuration option to determine how an instance provided by a DI container is shared in the system. It is represented by enum ObjectScope in Swinject.

The object scope is specified with inObjectScope method when you register a pair of a service type and component factory. For example:

container.register(Animal.self) { _ in Cat() }
    .inObjectScope(.container)

The object scope is ignored if the factory closure returns a value type because its instance is never shared per the Swift specification.

Built-in scopes

Transient

If ObjectScope.transient is specified, an instance provided by a container is not shared. In other words, the container always creates a new instance when the type is resolved.

If this scope is specified, circular dependencies resolution will not work properly.

Graph (the default scope)

With ObjectScope.graph, an instance is always created, as in ObjectScope.transient, if you directly call resolve method of a container, but instances resolved in factory closures are shared during the resolution of the root instance to construct the object graph.

Container

In ObjectScope.container, an instance provided by a container is shared within the container and its child containers (see Container Hierarchy). In other words, when you resolve the type for the first time, it is created by the container by invoking the factory closure. The same instance is returned by the container in any succeeding resolution of the type.

This scope is also known as Singleton in other DI frameworks.

Weak

In ObjectScope.weak an instance provided by a container is shared within the container and its child containers as long as there are other strong references to it. Once all strong references to an instance cease to exist, it won't be shared anymore and new instance will be created during next resolution of the type.

Above holds for reference types - value types are not shared in this object scope.

Custom Scopes

Custom object scopes can be defined like this:

extension ObjectScope {
    static let custom = ObjectScope(storageFactory: PermamentStorage.init)
}

Instances in .custom scope will be shared in the same way as in .container scope but can be discarded as needed:

container.resetObjectScope(.custom)

After scope is reset, container will create a new instance first time the type is resolved and will share this instance in any succeeding resolution of the type.

Behavior of custom scopes can be additionally modified by providing different storageFactory, or writing custom implementation of ObjectScopeProtocol protocol.

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